Getting things into the cell: Mediated Transport

There are different methods of getting things into and out of cells. The method of movement depends on the chemical structure, size, and polarity of the molecule. 

In order to best understand this, lets first take a look at the structure of a plasma membrane. 

The plasma membrane can best be described by the Fluid Mosaic Model. The plasma membrane or cellular membrane is a double-layered membrane that serves to protect the cell from its external environment. It is mostly composed of phospholipids, which have polar head groups facing the outside (since they are hydrophilic or water-loving) and non-polar hydrophobic (water-hating) tails made up of Fatty Acids. 

Now in order for a molecule to get into or out of the cell, it needs to cross this plasma membrane. There are several different methods to accomplish this:

1. Simple Diffusion: this allows you to cross the membrane by simply diffusing through it. In order to do this however, the molecule must be small enough and it cannot be polar. 
2. Protein Channels: this are areas in the lipid bilayer membrane that allow small polar molecules to cross the membrane. 
3. If the molecule is too polar and/or large and is unable to enter the cell via simple diffusion, then  a carrier is needed. Carriers are used to literally move the molecule from one side of the cell to the other. 

Most molecules will require a carrier to enter the cell. This is called Mediated Transport and there are different versions of this movement. It can be active or passive depending on the factors and conditions. Active Mediated Transport moves a molecule against its concentration gradient into the cell using the power of ATP (Adenosine Triphosphate). Passive Mediated Transport is... passive. 

The take home message in all of this is that if a molecule is polar then it can only cross the membrane by either a channel or a pump. A great way to learn through books, blogs, and tons of apps is through the new Kindle Fire. Shop Amazon - All - New Kindle Paperwhite